1. Field of the Invention
This invention relates to an engine control system, and more particularly relates to an engine control system which can remove stains, soot, particulates and so on from an exhaust gas recirculating passage.
2. Description of the Related Art
In order to detoxify exhaust gases, an engine is generally provided with not only filters, catalyzers and so on which constitute an exhaust emission control unit, but also an exhaust gas recirculating (EGR) device recirculating exhaust gases to a combustion chamber, thereby preventing NOx from being discharged to the exterior.
For instance, Japanese Patent Laid-Open Publication No. 2002-309,987 (called the “Reference 1”) describes an exhaust gas denitrifying device which includes a NOx absorbent in an engine exhaust passage and an exhaust gas recirculating unit (EGR) in order to recirculate exhaust gases to a combustion chamber. The NOx absorbent usually absorbs NOx, and discharges NOx when an air-fuel ratio is low in the combustion chamber. In other words, NOx is reduced using carbon monoxide (CO) and hydro carbon (HC). Conversely, in order to reduce the air-fuel ratio, an EGR control valve is opened in order to recirculate exhaust gas, to decrease an amount of intake air, and to increase an amount of fuel to be injected, thereby preventing an engine output torque from being changed.
Further, Japanese Patent Laid-Open Publication No. 2000-186,631(Reference 2) describes an internal combustion engine, which includes an EGR device in order to recirculate combustion gases into an EGR passage of the EGR device, and in order to burn SOF (soluble organic fractions), soot and so on accumulated in the EGR device when the engine stops operating.
In Reference 1, a bypass having a large quantity EGR valve is utilized together with an ordinary EGR passage when the engine is operating at a low air-fuel ratio. Recirculation of a large amount of exhaust gases reduces NOx to be discharged, so that a NOx occluding catalyzer may be downsized. However, during the engine operation with the low air-fuel ratio, EGR gases contain a lot of unburned gas (HC) and soot, which stick on and contaminate an EGR pipe, an EGR cooler, an EGR valve, an intake manifold and so on. As a result, the EGR valve will fail to quickly open or close, or the EGR pipe will be clogged.
In the internal combustion engine with the EGR device in the Reference 2, gases burnt by the combustion-based heater are introduced into the EGR passage when the engine stops operating, so that SOF, soot or the like accumulated in the EGR device will be burnt. However, the combustion-based heater tends to enlarge the EGR device, which is difficult to be mounted on a vehicle. Further, since the EGR device is cleaned when the engine stops operating, hot exhaust gases may be emitted via an exhaust pipe even during the non-operation of the engine.